It is known in the art to arrange an internal combustion engine to perform the combustion of a fresh charge of fuel in two separate sequential working cycles having separate stages of combustion and each yielding an output of work to the engine output shaft.
The general method of operation of such engines, which I refer to as staged combustion engines, involves the admission of a rich air-fuel charge to an engine combustion chamber wherein it is passed through a first stage working cycle including compression, combustion and expansion steps. This cycle results in the delivery of power to the engine output shaft and leaves a residual charge of combustion products and incompletely burned combustibles. To this charge, air is added to form a second and preferably somewhat lean combustible mixture heavily diluted with burned gases. This mixture is passed through a second stage cycle of compression, combustion and expansion, again yielding work to the engine output shaft.
Such a process is capable of being performed sequentially in the same engine combustion chamber. However, it is presently believed preferable to utilize separate combustion chambers of the same engine for performance of the two combustion cycles and to transfer the products resulting from the first stage combustion cycle to the second stage combustion chamber through a connecting interstage passage, conduit or manifold in which the necessary secondary combustion air is added.
Various forms of staged combustion engines have been proposed. Among these are the arrangements disclosed in U.S. Pat. Nos. 2,113,601 and 2,113,602, both granted April 12, 1938 to N. P. Pratt. Additional forms and methods of operation of such engines are disclosed in my copending U.S. patent application Ser. No. 282,390, filed Aug. 21, 1972.
FUEL-AIR EXPERIMENTAL WORK WITH STAGED COMBUSTION ENGINES OPERATING WITH GASOLINE FULE-AIR MIXTURES, I have preferred to utilize conventional spark ignition for initiating first stage combustion. However, I have found that satisfactory engine operation may be obtained under properly controlled conditions utilizing either compression ignition or conventional spark ignition for initiation of second stage combustion.
Both methods of operation have been shown to require some degree of temperature control of the interstage gases in order to maintain engine operation and efficiency under the varying loads and speeds required of automotive type engines. For example, my copending U.S. application Ser. No. 282,390 discloses that interstage gas temperatures must be controlled within predetermined ranges which are functions of other engine variables such as engine speed, load and second stage compression ratio, in order to realize efficient timing of second stage ignition when compression or auto-ignition is utilized. The use of spark ignition in the second stage reduces the criticality but does not eliminate the need for interstage temperature control; since, if the temperature of gas admitted to the second stage is too low, misfiring may occur; while if the temperature is too high, pre-ignition will occur, resulting in overadvanced timing and inefficient operation. In addition, some degree of interstage temperature control is necessary to prevent inefficient thermal reaction in the interstage manifold.